The terms “biomass” refers to any non-fossilized, i.e., renewable, organic matter. The various types of biomass include plant biomass (defined below), animal biomass (any animal by-product, animal waste, etc.) and municipal waste biomass (residential and light commercial refuse with recyclables such as metal and glass removed).
The conversion of these inexpensive biomass materials have been studied extensively for use as a feedstock for fermentation products. In order for the microorganisms to use these biomass feedstocks, the feedstocks need to be broken down into sugars which may be easily used by the microorganisms.
Pretreatment of cellulosic biomass has been regarded as a necessary step to allow accessibility of cellulolytic enzyme to its substrates, cellulose and hemicellulose within the rigid cell wall structure. Many pretreatment technologies have been developed over years to reduce the recalcitrance of cellulosic biomass structure and most of them involve some form of chemical treatment in a liquid phase under high temperature and high pressure conditions. Sulfuric acid is one of the most commonly added chemicals because it is less expensive compared to other chemicals such as base or solvent and it can result in an effective solubilization of hemicellulose.
Concentrated sulfuric acid can be utilized to solubilize both cellulose and hemicellulose to monomer sugars, however the process normally suffers from low hydrolysis yield due to formation of large amount of degradation products and subsequently very low fermentation yield because most of these degradation products are very inhibitory to fermentation organisms. Therefore, most of acid pretreatment is conducted at low concentrations of sulfuric acid. However, significant drawbacks exist. First, the hemicellulose fraction can be readily hydrolyzed to monomeric sugars by dilute acid. However, the cellulose and lignin fractions remains almost unaltered and significant amount of enzyme is normally needed to break down the insoluble cellulose fraction in an additional enzymatic hydrolysis step. Secondly, dilute acid pretreatment is normally performed in a liquid phase reactor under high temperature and high pressure conditions. Sufficient amount of water is required or a presoaking step is necessary to ensure the best mixing of acid catalyst and biomass. Because of the strong corrosion effects of dilute acid under these conditions, very expensive construction material for reactors is normally required. Finally, the acid must be neutralized before an enzymatic hydrolysis or fermentation step can take place. Formation of large amount of gypsum could be problematic to downstream processing when neutralized with inexpensive calcium hydroxide. Accordingly, there is a need for a more efficient and inexpensive methods of making lignocellulosic materials suitable for producing fermentable sugars for the biosynthetic production of various fermentation products.